Radiation detector



Aug. 1, 1961 E. J. STERNGLASS I 9 RADIATION DETECTOR Filed Feb. 20, 1956IO H 4 18 Source of 33 3 Radiation 5 5 24 E El 12 5 14 Fig.l. ii 49 Q IFig.2. 'w ;50 Radiation 54 Secondary Electrons Delta Ray InsulatingLayer Low Atomic Scattering Layer Number Layer 60 f Salrce of a x 66 70Ra iafion if 64 ea 72 t 5 IOI '84 sa 92 '96 Fig.3.

vW WQF VQN" WITNESSES INVENTOR Ernest J. Srernglass ATTORNEY UnitedStates Patent 2,994,773 RADIATION DETECTOR Ernest J. Sternglass,Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 20, 1956, Ser.No. 566,467 16 Claims. (Cl. 250-833) This invention pertains toelectrical discharge devices and, more particularly to radiationdetectors.

A widely used method for counting all types of radiation includingnuclear particles when short time resolution is desired is ascintillation type of counter. Such a counter employs a phosphor whichemits light flashes when radiation or nuclear particles impinge thereon,and a photomultiplier tube to convert the light flashes to electricalpulses and amplify the electrical pulses so they may be detected bywell-known amplifier and counting circuits. When organic compounds areused as the phosphors, the time resolution of the counter can be reducedto the order of a few millimicroseconds. This interval is satisfactoryfor many purposes, but for other purposes, such as a study of nuclearprocesses associated with fision and fusion reactions, it is desirableto shorten the interval even further.

Another problem which arises in the use of scintillation counters is theneed for an extremely sensitive photocathode in the photomultipliertube. This problem is further complicated by the present trend towardeven larger photocathode surfaces. At present, photocathode surfaces canonly be prepared by a complex series of critical manufacturing stepswhich result in a very high manufacturing cost for the photocathodesurface and thus a high cost for the scintillation counter.

Accordingly, it is an object of this invention to provide a radiationdetector having a cathode which converts radiation directly into asubstantial number of slow electrons which can be multiplied by variouselectron multiplying means.

Another object of this invention is to provide a cathode which convertsradiation directly into a substantial number of slow electrons and whichhas a very short resolution time.

Another object of this invention is to provide a cathode which convertsradiation directly into a substantial num ber of slow electrons whichcan be multiplied by means of a transmission type of electronmultiplier.

These and other objects and advantages of this invention will be moreeasily understood by those skilled in the art from the followingdetailed description of a preferred embodiment thereof when taken inconjunction with the attached drawing, in which:

FIGURE 1 is a longitudinal section of a radiation detector constructedin accordance with this invention employing a transmission type ofelectron multiplier;

FIG. 2 is an enlarged cross section of a portion of the cathode shown inFIG. 1; and

FIG. 3 is a longitudinal section of another embodiment of this inventionusing a conventional type of electron multiplier.

The radiation detector of this invention consists generally of anenvelope with a cathode 16 mounted adjacent one end of the envelope. Thecathode 16 converts the radiation directly to slow electrons which aremultiplied by means of an electron multiplier which is also "iceenclosed within the envelope 10. When the word radiation is used herein,it is meant to include alpha and beta particles, as well as electrons,mesons and other forms of nuclear radiation. Neutrons may also bedetected by adding boron or barium compounds, such as lithium borate, tothe cathode 16. The evacuated envelope 10 may be formed of any desiredmaterial such as glass or metal and is preferably tubualr in shape withend portions 12 and 14 at each end thereof. The end portion 12 should beformed of a material through which the radiation which is to be detectedcan easily pass, such as glass or aluminum, while the end 14 may beformed of the same material as the tubular portion of the envelope. Thecathode 16 is mounted on the inner surface of the end 12 with anelectrically conductive layer 18, such as a thin film of aluminum whichwill allow radiation to pass through, interposed between the cathode 16and the inner surface of the end portion 12. The exact construction ofthe cathode 16 will be described in greater detail below. Placed behindthe cathode 16 are a series of dynodes 20, '22 and 24 of thetransmission type. These dynodes are capable of receiving primaryelectrons on the surface facing the cathode 16 and emitting a largernumber. of secondary electrons from the other side of the dynode. Inthis manner, electrons emitted from the cathode 16 are multiplied sothat the signal transmitted by the radiation detector is sufiicientlylarge that it can easily be amplified and counted by well-knownamplifying and counting circuits. This type of electron multiplier whichis referred to as a transmission type electron multiplier is moreparticularly described in the applicants copending application entitledElectron Discharge Device, Serial No. 434,467, filed June 4, 1954, nowUS. Pat. No. 2,905,844, issued Sept. 22, 1959, and assigned to the sameassignee as this invention, and which is incorporated in thisapplication by reference.

The cathode 16 as well as the dynodes 20, 22 and 24 should be providedwith a source of positive potential which increases in magnitude betweenthe cathode 16 and the dynode 20 and each succeeding dynode so that theelectrons which are emitted from the cathode 16 will be accelerated tothe dynode 20 and each succeeding dynode. This source of potential isprovided in FIG. 1 by a battery 26, the negative terminal of which isconnected to the conductive layer 18 by means of a lead 28. A pluralityof series connected resistors 32, 34, 42 and 44 are connected betweenthe positive and negative sides of the battery 26 as shown in FIG. 1.The dynode 20 is connected to a point between resistors 32 and 34 bymeans of a lead 30, while the dynode 22 is connected to a point betweenthe resistors 42 and 44 by means of a lead 40. The dynode or anode 24 isconnected to the resistor 44 by means of a lead 49 and a resistor 46.The positive side of the battery 26 is connected to the junction of theresistors 44 and 46 by means of a lead 48, and a lead 50 connects thepositive side of the battery 26 to ground. The signal from the radiationdetecting device is transmitted to the associated amplifying andcounting equipment (not shown) by means of a lead 51 which is connectedto the lead 49 between the dynode 24 and the resistor 46. In thismanner, cathode 16, dynodes 2t), 22 and 24 have progressively increasingsteps of positive potential, with respect to the cathode 16 so as toaccelerate electrons from cathode 16 to the first dynode 2G and eachsucceeding dynode. Although equal steps .travel a shorter distance.

. V 3 of volta-ges between the cathode 16 and the dynodes 20, 22 and 24are shown, it may be desirable to operate some of the dynodes at highervoltages.

Shown in FIG. 2 is an enlarged cross section of the cathode 16consisting of three layers 52, 54 and 56 which are in contact with eachother. The layer 52 is a layer of material having a low atomic number,so as not to scatter the delta rays which are formed therein, such asgraphite or beryllium, and it should have a thickness no greater thanthe range of the delta rays which are released when radiation impingeson the cathode 16. This thickness will vary between to 10- cm. inthickness depending on the type of radiation being detected.

The thickness of the layer 52 should be no greater than the range of thefastest delta ray released because a greater thickness would stop thosedelta rays formed in the portion of the layer which exceeds their range.A thickness less than the range of the fastest delta ray could be usedbut fewer delta rays would be formed by the radiation impinging on thecathode since it would Thus, the thickness of the layer 56 should beequal to or less than the range of .the fastest delta ray formed by theradiation because this thickness will give the best results whileabsorbing .a minimum of the radiation being detected. For example, if itis desired to detect 300 million electron volt (mev.) protons which willform .5 mev. delta rays when they strike the layer 52, the layer shouldhave a thickness equal to approximately .200 gram/per cm. The layer 54is formed of a material having a high atomic number which is also aconducting material such as platinum or gold, so that it will scatterthe delta rays which are formed in the layer 52 and also serve as a gridin case it is desired to omit layer 18. The layer 54 should berelatively thin on the order of 10* to 10- cm. so that the delta rayscan easily pass through it and not be stopped. Of course, the thicknessof the layer 54 should be matched to the energy of the delta particlesreleased by the particles being detected, that is, for higher energydelta particles the layer 54 should be thicker than for lower energydelta rays. The final layer 56 is composed of an insulating materialsuch as an alkali-halide, alkali earth oxides or metal oxides, which hasthe property that slow secondary electrons can travel large distances init before losing their ability to escape.

In order to have this property the layer 56 should have a long mean freepath for secondary electrons and a large energy gap between the filledvalence band and the conduction band of each atom. This will result in alarge yield of secondary electrons which can diffuse through asubstantial thickness of layer 56 without losing a large amount ofenergy. Thus, a large number of secondary electrons will be formed whichcan easily escape from the back side of layer 56. The material shouldalso have good crystallinity and be easily fabricated into thin films byevaporation or other means.

Some examples of materials having these properties are potassiumchloride, sodium chloride, aluminum oxide, magnesium oxide and magnesiumfluoride. The layer 56 should have a thickness equal to a few diffusionlengths of the secondary electrons in the particular material used andis approximately 10* to 10- cm. in thickness. The various layers 52, 54and 56 may be fabricated by any well known method such as the depositingof the layers, one on the other, as described in the applicationreferred to above.

The cathode described above will convert the radiation which impinges onit to a substantial number of slow electrons which will be emitted fromthe back or the right hand side of the layer 56 as shown in FIG. 2. Whenthe radiation strikes the cathode 16, the layer 52 will convert it to afew fast delta rays which will easily escape from the layer 52. Whenthese delta rays strike the layer 54, they will be scattered and escapeinto the layer 56 of insulating material at a large angle. The

insulating material will, in turn, convert the delta rays to a largenumber of slow electrons which can easily escape from the back of thelayer 56. For example, when 300 mev. protons strike the layer 52,approximately 1 to 10 slow electrons, but averaging three, are emittedfrom the back side of the insulating layer 56. These three electronswill be greatly multiplied by the transmission type of electronmultiplier placed behind the cathode 16 shown in FIG. 1 so that thepulses transmitted from the detecting device will correspond to 10 to 10electrons at anode 24.

Since the number of delta rays per cm. of path is inversely proportionalto the square of the velocity of-the particle striking the layer 52, theyield of slow electrons will be much greater for particles having alower velocity than the 300 mev. proton example given above. In additionto this, the yield of slow electrons is directly proportional to thesquare of the charge carried by the particle striking layer 52. Thus,alpha particles of the same velocity as the proton example above willyield four times the number of slow electrons as are yielded by theproton because of the higher charge carried by the alpha particles.

Shown in FIG. 3 is a modification of the radiation detecting deviceshown in FIG. 1. In this embodiment, the electrons emitted by thecathode 16 are multiplied by the conventional type of electronmultiplier in which the primary electrons impinge on one surface of thedynodes and the secondary electrons are emitted from the same surfaceand transmitted to the next dynode. This device utilizes an evacuatedenvelope 60 similar in construction to the envelope It in FIG. 1 withthe cathode 16 mounted adjacent one end thereof. Also mounted inside theenvelope 60 is a plurality of dynodes 62-72. These dynodes are providedwith a source of progressively increasing positive potential so thatslow electrons emitted from the cathode 16 will be accelerated from thesuccessive dynodes to the final dynode 72. This positive potential isprovided by a battery 76, the negative terminal of which is connected tothe cathode 16 by means of a lead 74. A plurality of series connectedresistors 78, 82, 86, 90, 94 and 98 are connected between the positiveand negative terminals of the battery 76. The dynode 62 is connected toa point between the resistors 78 and 82 by means of a lead and eachsucceeding dynode is connected to a similar point between the remainderof the resistors by means of leads 84, 88, 92 and 96. The last dynode oranode 72 is connected to one end of the resistor 98 through a lead 101and a resistance 100. The positive terminal of the battery 76 isconnected to the common junction of resistors 98 and by means of a lead162 and to ground by a lead 104. The signal from the detector istransmitted by a lead 103 which is connected .between the dynode 72 andthe resistance 100.

The operation of this embodiment of the invention is the same as thatdescribed for the embodiment shown in FIG. 1. The only difference is inthe type of electron multiplier used for multiplying the electronsemitted from the cathode 16. The embodiment of the detector shown inFIG. 3 utilizes a multiplier in which the secondary electrons areemitted from the same side of the dynode from the primary electronsimpinged. The secondary electrons are accelerated from one dynode to thenext dynode by means of progressively increasing steps of positivepotential between the various dynodes.

In addition to the use of this invention as a radiation detection deviceas described above, the embodiment of the invention shown in FIG. 1could easily be modified to serve as an imaging device. In order tomodify this embodiment to serve as an imaging device, it would only benecessary to substitute a phosphor screen in place of the last dynode24. The phosphor screen should be mounted directly on the inner surfaceof the end closure 14, which should preferably be transparent. Thus, theexact shape of the radiation striking the cathode 16 could be viewed onthe phosphor screen.

While only two embodiments of this invention are shown, manymodifications and additional embodiments thereof will occur to thoseskilled in the art within the broad spirit and scope of this invention,and, accordingly, it should be limited only as required by the priorart.

I claim as my invention:

1. A radiation detector comprising an evacuated envelope, a cathodemounted in said envelope, said cathode including a first materialcapable of directly producing delta rays therein upon impingement ofradiation, a second material capable of scattering said delta rays, anda third material capable of converting each of said delta rays to atleast one slow electron emitted by said third material, said materialsbeing disposed adjacent one another, respectively, andelectron-multiplying means mounted within said envelope.

2. A radiation detector comprising an evacuated envelope, a cathodemounted within said envelope adjacent a surface thereof, said cathodecomprising a first material disposed adjacent said surface and capableof converting radiation when impinging thereon directly into delta rays,a second material capable of scattering said delta rays and disposedinwardly of said first material in contact therewith, and a thirdmaterial capable of converting said delta rays into emitted electronsand disposed inwardly of said second material in contact therewith; andelectronmultiplying means mounted within said envelope.

3. A radiation detector comprising an evacuated envelope, anelectron-multiplying means mounted within said envelope, and a cathodemounted within said envelope and juxtaposed to said multiplying means,said cathode including a first material arranged for impingement byradiation and capable of converting said radiation directly into deltarays, a second material capable of scattering said delta rays, and athird material capable of converting said delta rays into electronshaving an energy level capable of being multiplied by saidelectron-multiplying means, said materials being adjacent to oneanother, respectively.

4. A radiation detector comprising an evacuated envelope,electron-multiplying means mounted within said envelope, and aradiation-receiving cathode mounted within said envelope and juxtaposedto said electronmultiplying means, said cathode including means forconverting said radiation directly into delta rays and additional meansfor scattering said delta rays and for converting said delta rays intoelectrons capable of being multiplied by said electron-multiplyingmeans.

5. A cathode for a radiation converting, electronic discharge device,said cathode comprising a laminated structure having a first materialcapable of converting said radiation directly into delta rays, a secondmaterial for scattering said delta rays, and a third material capable ofemitting electrons when exposed to said delta rays.

6. A cathode for a radiation converting, electronic discharge device,said cathode comprising means for receiving said radiation and forconverting said radiation directly into delta rays, additional means forreceiving and scattering said delta rays, and an electron-emitting meansfor receiving said scattered delta rays and for converting said deltarays into emitted electrons.

7. A cathode for an electronic discharge device arranged for convertingradiation into electronic emission, said cathode comprising a firstmaterial selected from the group including carbon and beryllium, asecond material selected from the group including platinum and gold, anda third material selected from the group including alkali halides metaloxides and alkali oxides, said materials being disposed adjacent oneanother, respectively.

8. A cathode for an electronic discharge device arranged for convertingradiation into electronic emission, said cathode comprising a firstmaterial including an element of relatively lower atomic weight disposedto receive said radiation, said first layer being capable of 6converting said radiation directly into delta rays, a sec ond layerincluding an element having a relatively higher atomic weight disposedto scatter said delta rays, and an electron-emitting third materialdisposed to convert said delta rays into electronic emission, saidmaterials being disposed adjacent to one another, respectively.

9. A radiation detector comprising an evacuated envelope,electron-multiplying means mounted within said envelope, and anelectron-emitting cathode mounted within said envelope in juxtaposedrelation to said multiplying means, said cathode enclosing a first layerhaving an element of relatively lower atomic weight disposed to receivesaid radiation and capable of converting said radiation directly intodelta rays, a second material having an element of relatively higheratomic weight for scattering said delta rays, and an electron-emittingthird material capable of converting said delta rays into electronicemission, said materials being disposed adjacent one another,respectively.

10. A radiation detector comprising an evacuated envelope, anelectron-multiplying means mounted within said envelope, and anelectron-emitting cathode mounted within said envelope in juxtaposedrelation to said multiplying means, said cathode having a first materialcapable of converting said radiation directly into delta rays, a secondmaterial for scattering said delta rays, and an electron-emitting thirdmaterial capable of converting said delta rays into emitted electronsand of multiplying the electrons thus formed, said materials beingdisposed adjacent one another, respectively.

11. A neutron detector comprising an evacuated envelope, a cathodemounted in said envelope, a neutronreactive material supported adjacentsaid cathode, said material being capable of emitting charged radiationupon impingement of said neutrons, said cathode including a firstmaterial capable of converting said radiation when impinging thereondirectly into delta rays, a second material capable of scattering saiddelta rays and a third material capable of converting each of said deltarays into at least one slow electron emitted by said third material,said materials being disposed adjacent one another respectively, andelectron multiplying means mounted within said envelope.

12. A cathode for a neutron sensitive electronic discharge device, saidcathode comprising a material capable of emitting charged radiation onimpingement of neutrons and a laminated structure having a firstmaterial capable of converting said radiation directly into delta rays,a second material for scattering said delta rays and a third materialcapable of emitting electrons when exposed to said delta rays.

13. A cathode for a radiation-converting, electronic discharge device,said cathode comprising means for receiving said radiation andconverting said radiation directly into electrons, and means fordirectly multiplying said converted electrons into electrons of lowerenergy.

14. A cathode for a radiation-converting, electronic discharge device,said cathode comprising means for receiving said radiation andconverting said radiation directly into electrons, and means forconverting said electrons into emitted electrons of lower energy, saidlastmentioned means also being formed to directly multiply said emittedelectrons.

15. A cathode for a radiation-converting, electronic discharge device,said cathode comprising means for receiving said radiation andconverting said radiation directly into electrons, means for convertingsaid electrons into emitted electrons or" lower energy, and means fordirectly multiplying said emitted electrons.

16. A cathode for a neutron sensitive electronic dis charge device andcapable of emitting electrons in response to neutron excitation, saidcathode comprising means for receiving and converting said neutrons intoReferences Cited in the file of this patent UNITED STATES PATENTSSheldon Mar. 16, 1954 Victoreen Feb. 13, 1940 Teal Apr. 9, 1940 Kallmannet a1. Jan. 20, 1942 8 Kallmann Sept. 29, 1942 Kallrnann Mar. 14, 1944Sheldon Oct. 17, 1950 Sheldon June 5, 1951 Marshall et a1. Sept. 30,1952 Sheldon Dec. 14, 1954 Sheldon May 22, 1956 Botden et a1 May 29,1956 Jacobs et al. June 9, 1959

